A temperature sensor which uses a temperature sensing element whose resistance value changes according to the temperature has been in widespread use in order to measure the temperature of automotive exhaust gas, a water heater, a boiler, a multifunction microwave oven, a stove and the like. As shown in FIG. 11, a temperature sensor element (hereinafter referred to simply as a sensor element) used in this temperature sensor comprises a temperature sensing element 2 provided with a pair of electrodes 3, a lead wire 4 connected to each of the electrodes 3 in pair, and a covering material which seals the temperature sensing element 2. The covering material is made of heat-resistant crystalline glass, amorphous glass or the like. When the sensor element is installed in an electric oven (an electrical degree sensor), a radiant heater, a combustion appliance, an exhaust gas purifier and the like, the sensor element is used by being housed in a metallic protective tube having a high sealing performance in order to protect the sensor element from vibrations, external forces, combustion gas and the like.
According to Patent Document 1, the oxygen partial pressure around the temperature sensing element 2 sometimes varies due to the oxidation of a metallic protective tube which constitutes a temperature sensor when the use environment temperature becomes not less than 750° C. and as a result of this, the composition of the temperature sensing element 2 varies and the temperature-resistance value characteristic becomes unstable. Hence, Patent Document 1 proposes that a vibration-resistant filler with porosity of 30 to 70% be filled in the metallic protective tube.
Patent Document 1 adopts a method which involves simultaneously inserting a fluid vibration-resistant filler and a temperature sensor element into a metallic protective tube, burying both, and hardening the fluid vibration-resistant filler by heating. Although due to the volatilization of a binder during the hardening of the vibration-resistant filler bubbles may remain after hardening, it is impossible to check the existence of bubbles. Furthermore, because the vibration-resistant filler during the insertion into the metallic protective tube is in a powdery state, the filling density of the vibration-resistant filler in the metallic protective tube is not sufficient. Therefore, it cannot be said that a temperature sensor which is obtained has sufficient reliability against variations in oxygen partial pressure, and the accuracy of detected temperatures is unstable.